Links for 2015-03-26

Within the context of a distributed system, you cannot have exactly-once message delivery. Web browser and server? Distributed. Server and database? Distributed. Server and message queue? Distributed. You cannot have exactly-once delivery semantics in any of these situations.

At a recent call, Neha said “The most confusing behavior we have is how producing to a topic can return errors for few seconds after the topic was already created”. As she said that, I remembered that indeed, this was once very confusing, but then I got used to it. Which got us thinking: What other things that Kafka does are very confusing to new users, but we got so used to them that we no longer even see the issue?

This is the second part of our guide on streaming data and Apache Kafka. In part one I talked about the uses for real-time data streams and explained our idea of a stream data platform. The remainder of this guide will contain specific advice on how to go about building a stream data platform in your organization.

The JVM by default exports statistics by mmap-ing a file in /tmp (hsperfdata). On Linux, modifying a mmap-ed file can block until disk I/O completes, which can be hundreds of milliseconds. Since the JVM modifies these statistics during garbage collection and safepoints, this causes pauses that are hundreds of milliseconds long. To reduce worst-case pause latencies, add the -XX:+PerfDisableSharedMem JVM flag to disable this feature. This will break tools that read this file, like jstat.